1. Technical Field
This disclosure relates to lightweight thermionic microengines for aerial vehicles. More specifically, the lightweight thermionic microengines disclosed herein provide a propulsion system for an aerial craft, such as a drone or a UAV (unmanned aerial vehicle), which uses lightweight thermionic microengines to convert energy from chemical based fuel into electricity used to control flight operations the craft.
2. Description of the Related Art
Conventional airplanes, helicopters, and other flying craft have relied on jet engines or internal combustion engines to create the thrust necessary for powered flight. These large aerial vehicles have transformed the world by flying cargo and passengers from one place to another. However, these large aerial vehicles require substantial amounts of fuel in order to produce the thrust necessary to take off and maintain flight.
More recently, unmanned craft have begun taking on flight roles traditionally performed by manned craft. For example, UAVs or drones are used by militaries today to perform reconnaissance, launch weapons, or monitor battle situations without exposing the operator to risk. At the same time, UAVs and drones have been used by hobbyists, cartographers, photographers, civil engineers, treasure hunters, and a host of other professional and amateur operators to provide a point of view of a particular area or object that would be prohibitively expensive, difficult, or dangerous to obtain by a manned craft, such as an airplane or a helicopter.
In virtually every situation, a more lightweight aerial vehicle is desirable over a heavier aerial vehicle because lighter aerial vehicles require less thrust to achieve flight, all other considerations being equal. Since the amount of thrust produced is directly proportional to the energy expended, the lightest aerial vehicle that will perform a particular task is typically best. Since the weight of a propulsion system is a significant fraction (20-40%) of the total weight of a UAV, it is desirable that the propulsion system weigh as little as possible. Further, it is desirable for the UAV fuel or energy storage device to have a high specific energy content, or in other words, a relatively high energy output to mass ratio. Further, the less energy that is expended for propulsion of an UAV from chemical based fuels or electricity, the longer an aerial vehicle can maintain flight and the farther the aerial vehicle can travel without refueling.
Engines derive energy from the heat of combustion of chemical fuels such as hydrocarbons (natural gas, butane, and gasoline) or hydrogen. The specific energy of hydrocarbon fuels (say, butane) is 45 MJ/kg (12.5 kWh/kg), and that of hydrogen is 140 MJ/kg (38.9 kWh/kg). Specific energy of chemical fuels are about hundred times greater than state-of-the-art battery technologies. Lithium polymer (LiPo) batteries, which are commonly used in drones, have low specific energies (<0.17 kWh/kg). However, engines such as jet engines and internal combustion engines, utilized to convert energy in chemical fuels to propeller power are heavy and cannot be scaled down in size because of poor reliability and high cost of manufacturing.
In order to reduce weight, many UAVs and drones use batteries or sodium borohydride (NaBh4) fuel cells to provide the power necessary to create sufficient thrust for flight. However, batteries are also subject to a host of limitations. For example, although advanced batteries such as lithium-ion, lithium polymer (LiPo), and other batteries provide power suitable to create enough thrust for a UAV or a drone to fly, batteries in general are expensive, have low specific energies, low power densities, short cycle lives, long charging times, are sensitive to adverse weather and humidity conditions, can be toxic, and, in many cases, have a generally adverse effect on the environment in terms of manufacture and disposal. Many UAV and drone operators find that these limitations limit their ability to use or their enjoyment in using and operating a UAV or drone. Further, these limitations provide powered flight for a disappointingly short amount of time, even with very small, often inadequate payloads.
Accordingly, it is one object of this disclosure to provide lightweight thermionic microengines and robust propulsion power system for aerial vehicles such as UAVs and drones. It is another object of this disclosure to overcome the limitations of conventional battery technology while providing thrust to an aerial vehicle by means of electric power. It is another object of this disclosure to enhance a UAV or drone operator's flight experience by providing a propulsion system that increases flight time and load capacity.
It is a further object of this disclosure to provide a propulsion system that does not rely on electrical power provided from an electrical grid. Yet another object of this disclosure is to increase the payload capacity of a drone that operates by means of electrical power.